Like any peace deal bringing to end decades of conflict the Good Friday agreement of 1998 in Ireland was a historic compromise that provided gain and pain for both sides. The support of the IRA and loyalist terror groups for the peace deal was secured by granting their prisoners early release from the top security Maze prison. In total 428 paramilitaries, including 143 prisoners serving life sentences for murder including some of the worst atrocities in the Troubles, were given early release from the Maze between 1998 and 2000.

But for Sinn Féin, one issue remained: the fate of the so-called "on the runs", the 187 fugitives still wanted for offences committed during the troubles. The reason Sinn Féin was so adamant the fugitives should get such assurances is that many of them have influence within the wider republican movement.

John Downey, for instance, is a friend of Sinn Féin MP Pat Doherty and a mentor to the party's rising star in the Irish parliament Pearse Doherty, with all three men coming from Co Donegal. A deal between the party and Tony Blair's government was reached and fugitives began receiving the secret "get out of jail letters" – notes that it has emerged continued to be sent after the coalition took office in 2010.

But what seems clear is that the existence of the "get out of jail" letters was not so widely known beyond British government ministers and Sinn Féin officials. Peter Robinson, the Northern Ireland Democratic Unionist first minister said he was kept in the dark until Tuesday, though Sinn Féin says the existence of the scheme itself was broadly known.

By announcing his judge-led inquiry into the letters for wanted IRA fugitives, David Cameron has provided a shield to help Peter Robinson, Northern Ireland's embattled first minister, ward off attacks from unionist hardliners.

It is no accident that the investigation headed by a judge into the letters will conclude at the end of May – by the time of its release, elections to local councils in Northern Ireland and the European electoral contest will be over. Robinson has got what he wants after having threatened to resign.

Sinn Féin will be watching developments carefully in case any judicial inquiry threatened to rescind the amnesty letters to IRA personnel, all of whom now support the peace process.Downey's arrest sent a chill down the spines of many influential ex IRA members including one former finance officer for the Provisionals. He refused to hold meetings in Northern Ireland during Downey's incarceration because he feared being arrested.

Any attempt to reverse the Old Bailey decision to free Downey or make the letters legally invalid would increase internal pressure on the Sinn Féin leadership in the same way as the DUP high command have been under over the past few days.

Other armed "actors" in 35 years of conflict in Northern Ireland will also be observing developments in this controversy. The paratroopers who killed unarmed civilians on Bloody Sunday, the police officers accused of "shoot-to-kill" operations against republicans in the 1980s and of course loyalist paramilitaries who don't have any "get out of jail" letters to protect them from arrest, will all be monitoring events.

If the letters' legality stands at the end of May their lawyers will argue that what applies to IRA members wanted for past Troubles crimes also applies to them. The victims of loyalist and state violence may wonder whether those who killed their loved ones will use these written assurances to argue that any cases against them are unfair, invalid and unjust.